If you have a bike in this city, at some point you're going to need to get off of it and leave it by its lonesome. As is the case with dogs and screaming children, there are some places a two-wheeled companion are not welcome. Bike racks, then, are a necessary part of bike life . . . and they're a lot safer than the random mesh fences and wooden railings we're forced to resort to when there are no vacancies anywhere else. With the student bike population out of town, however, we now have free rein of Boston's trails, and racks. From the standard upside-down U's to Brookline's ritzy bike-shaped racks, the best parking is now all yours. Take your bike for a spin down the Esplanade without worrying about dodging all the neon-pink-sports-bra-clad joggers, juiced-up lax bros, and — if it's anywhere near sunset — clouds of pot smoke wafting up from the docks.